This invention relates to a method of coating a thin liquid film on a substrate surface and apparatus for the coating method. For example, the coating method is suitable for use in forming a thin film of a metal oxide on a limited area of a surface of a relatively large substrate by a sol-gel method using a soltuion of a metal alkoxide.
Conventional wet coating methods include dip coating method in which the substrate is dipped into a coating liquid and then pulled out at a constant speed, flow coating method in which a coating liquid is flowed from an upper edge of the substrate so as to spread on the substrate surface and roller coating method in which the substrate is carried by a pair of rolls at least one of which is always wetted with the coating liquid.
However, in many cases and particularly in the cases of coating relatively large substrates difficulty or inconvenience is experienced in accomplishing desired coating by any of conventional wet coating methods. As to flow coating method and roller coating method, coating of only a selected area of a substrate surface has to be accomplished by first coating the entire area of the substrate surface and then removing the coating film in the unwanted areas by a troublesome technique such as etching, because it is impracticable to apply masking tape or the like to the unwanted areas. Besides, by either of these coating methods it is very difficult to form a coating film having a submicron and well controlled thickness.
For forming a submicron coating film by a wet coating method, dip coating method is generally accepted as most suitable. However, coating of a large-sized substrate by this method entails large-scale machinary for dipping the substrate into the coating liquid and then pulling out it. Moreover, transverse stripes are liable to appear in the coating film because in pulling out the substrate it is inevitable that vibration of the substrate causes oscillation of the liquid level. By this method it is very difficult to form a broad and uniformly thin coating film. Besides, coating of only a limited area of a substrate surface by dip coating method is troublesome and time-consuming because it is necessary to mask not only the unwanted areas of the substrate surface but also the entire area of the opposite surface or, alternatively, to first coat the entire areas of the both surfaces and then remove the coating film in the unwanted areas of the front surface and over the entire area of the opposite surface.